In marine seismic surveying as currently performed, a long array of airguns may contain forty or fifty or more airguns towed by a ship with an umbilical extending back from the ship along the length of the long array.
In such a towed array, it would be desirable (from the viewpoint of individual airgun control) to provide individual high-pressure hoselines extending back from the ship for feeding compressed air to respective individual airguns. Then, the ship could be equipped with an onboard shutoff valve for each individual hoseline. In the event of compressed air leakage due to rupture of a particular hoseline or due to non-closure of a particular airgun after firing, the onboard shutoff valve for the leaking hoseline could be closed. A computer program being used for assembling seismic data resulting from operating the array could be adjusted to account for a non-operating airgun in the array. Then surveying could proceed with minimal delay.
As a practical matter, when using such a large number of airguns in a towed array, it is not feasible to provide a multiplicity of individual high-pressure airhoses extending back from the ship to a multiplicity of individual airguns. Attempting to bundle such a large number of individual airhoses into an umbilical for a towed array would create an unduly bulky, large diameter, unwieldy umbilical. Such a bulky, heavy umbilical would be difficult to launch and to retrieve. Also, its relatively large diameter would cause increased towing stresses and more fuel consumption for the ship, due to undesirably large frictional drag resulting from a large-diameter umbilical being towed through the water.
Consequently, the umbilical is constructed with relatively few main high-pressure hoselines, each serving as a common air line for supplying numerous airguns. In many cases, only one common main high-pressure supply line is included in the umbilical depending on configuration of an array or of a sub-array. Connected to the common hoseline(s) at spaced intervals are smaller-diameter branch hoselines which extend from the common line to respective individual airguns or to airgun clusters.
With the use of a common airline, the occurrence of a non-closing airgun or a ruptured feeder line allows large volumes of compressed air continuously to escape from the common airline into a surrounding body of water. Such continual loss of compressed air from a common line is detrimental to or completely disrupts a seismic surveying operation due to loss of air pressure in the common line. Other airguns cannot operate properly with significantly reduced pressure in the common airline. Consequently the survey must stop until repairs are made to staunch the leak. The ship is stopped, and the airgun array is hauled on board for making the needed repairs. Such a stop for repairs in the midst of a marine seismic survey is wasteful of valuable time and may even curtail operations if the stop for repairs happens to occur during a "window of opportunity" when local weather conditions at sea may briefly be favorable for conducting the survey. Moreover, such loss of time is costly to a projected budget.